1. Field of the Invention
The present invention relates to an image recording apparatus, such as a laser printer, which is capable of printing characters and symbols on paper at high quality with a low noise.
2. Description of the Related Art
In recent days, a laser printer is widely used as an output device of the computer because of producing a high quality image with a low noise.
The inventors of the present invention know a laser printer which is capable of printing an image at a resolution of 300 dpi and a pace of 10 sheets per minute. However, a laser printer which has 600 dpi or more resolution is requested by many users who desire to print more image in the same period of time.
In a case that a printer having a printing density of 600 dpi is used, dot data for a dot density (printing density) of 300 dpi is reduced to a half in a vertical and a horizontal directions of the dot data.
To avoid this reduction, it is necessary to double the number of dots in the vertical and the horizontal directions.
Meanwhile, the rugged contour of the character printed at 600 dpi is not improved as compared with the character printed at 300 dpi.
It is certain that the high-quality image is allowed to be printed by printing the dot data for the printing density of 600 dpi at the density of 600 dpi. However, for achieving this, a capacitance of the video memory is required to be four times larger than the known printer which is capable of printing an image at 300 dpi.
As a result, the laser printer becomes more costly, and the laser printer cannot be applied to the existing application programs developed for a computer designed to output the dot data for the printing density of 300 dpi to the printer, disadvantageously.
To overcome the above-mentioned disadvantages, the apparatus disclosed in U.S. Pat. No. 4,847,641 is capable of printing 300-dpi data at a printing density of 600 dpi as a pseudo image. In a case that the dot pattern of a character to be printed coincides with a pre-stored pattern, the disclosed apparatus serves to change a spot size of a light beam or slightly move the location of the dot in the main-scan direction (by 1/3 dot, for example).
However, the disclosed apparatus serves to process a signal only in the main-scan direction, that is, the image quality is not enhanced in the sub-scan direction.